The invention relates to a ceramic pressure measuring cell.
In pressure measurement, absolute-pressure, relative-pressure and differential-pressure measuring cells are used for example. In the case of absolute-pressure measuring cells, a pressure to be measured is registered in absolute terms, i.e. as a pressure difference with respect to a vacuum. With a relative-pressure measuring cell, a pressure to be measured is picked up in the form of a pressure difference with respect to a reference pressure, for example a pressure which prevails where the sensor is located. In most applications, this is the atmospheric pressure at the place of use. Consequently, in the case of absolute-pressure measuring cells, a pressure to be measured is sensed in relation to a fixed reference pressure, the vacuum pressure, and in the case of relative-pressure measuring cells a pressure to be measured is sensed in relation to a variable reference pressure, for example the ambient pressure. A differential-pressure measuring cell senses a difference between a first pressure and a second pressure bearing on the measuring cell.
There are on the market ceramic pressure measuring cells with
a basic body
a diaphragm connected to the basic body to form a measuring chamber,
which during operation undergoes a deflection dependent on a pressure to be measured, and
an electromechanical transducer, which serves the purpose of registering the deflection of the diaphragm and making it accessible for further evaluation and/or processing.
Ceramic pressure measuring cells are advantageously used for pressure measurement, since ceramic pressure measuring cells have a measuring accuracy which is stable over a very long time. One reason for this is the strong ionic bonding of ceramic, which makes the material very durable and undergo virtually no aging in comparison with other materials, for example metals.
Ceramic pressure measuring cells have so far usually been designed as what are known as dry absolute- or relative-pressure measuring cells. A dry pressure measuring cell is not filled with a liquid which transfers a pressure, for example into a measuring chamber of the pressure measuring cell. The pressure measuring cell is restrained at an outer edge in a housing, with a seal interposed. A pressure to be measured is fed to the diaphragm through an opening in the housing or a process connection connected to the generally metallic housing. A pressure to be measured can only be fed directly to such measuring cells. The use for example of an upstream diaphragm seal is usually not envisaged in the case of these pressure measuring cells.
It is possible to feed the pressure to the measuring chamber via a small pressure tube which has been fitted, for example cemented or soldered, into the basic body. A soldered-in small pressure tube provides an elastomer-free, hermetically sealed joint for the pressure measuring cell. The small pressure tube and the measuring chamber, and also a pressure source, for example a diaphragm seal, arranged upstream of the small pressure tube, are filled with a liquid.
In the case of such liquid-filled pressure measuring cells, it is of advantage for their measuring accuracy that the volume of liquid is as small as possible. The reason for this essentially lies in a thermal expansion of the liquid as a function of temperature, which leads to a change in the internal pressure in the pressure measuring cell and consequently to a change in its measuring properties.
During the calibration of the pressure measuring cells, it is of advantage if the required volume of liquid is as identical as possible from one measuring cell to the other. The more the pressure measuring cells to be calibrated are the same, the less effort is required for example for determining the characteristic data of the measuring cells.
Pressure measuring cells which serve for measuring a pressure difference typically have two identically formed halves, which are each connected to a pressure source. In the case of these pressure measuring cells, it is important not only that each half requires as small a volume of liquid as possible but also that the required volume of liquid in both halves is as identical as possible. One reason for this is that different volumes result in a different change in volume in the two halves when there is a change in temperature. This can lead to considerable temperature-dependent measuring errors.
It is an object of the invention to specify a ceramic pressure measuring cell in which a pressure is fed to the measuring chamber by means of a small metallic pressure tube and in which a volume inside the measuring cell and the small pressure tube can be established as reproducibly as possible.
For this purpose, the invention comprises a ceramic pressure measuring cell with
a basic body
a diaphragm connected to the basic body to form a measuring chamber,
which during operation undergoes a deflection dependent on a pressure to be measured,
an electromechanical transducer, which serves the purpose of registering the deflection of the diaphragm and making it accessible for further evaluation and/or processing, and
a bore which penetrates the basic body
and into which a small pressure tube is soldered in a pressure-resistant and gastight manner,
and via which a pressure is introduced into the measuring chamber, and
a mechanical stop, by which a depth of penetration of the small pressure tube into the basic body is fixed.
According to a first embodiment, the stop is a shoulder which is formed onto the small pressure tube, extends radially outward and rests on an annular face of the basic body surrounding the bore.
According to a second embodiment, the bore has a portion facing the measuring chamber and a portion facing away from the measuring chamber, the portion facing the measuring chamber having an inside diameter which is smaller than an outside diameter of the small pressure tube and is an inside diameter of the portion facing away from the measuring chamber, and between the two portions there is an annular face, which forms the stop and on which the small pressure tube rests with an annular end face.
According to one embodiment, the solder is a glass solder.
According to another embodiment, the solder is a metallic hard solder and the ceramic basic body has a pre-metallization at a connecting point between the basic body and the small pressure tube.
According to a further embodiment, the solder is an active hard solder.
The stop achieves the effect that a depth of penetration of the small pressure tube into the basic body is precisely predetermined. In a corresponding way, a volume which displaces the small pressure tube in the bore in the basic body can be established in a reproducible manner. Together with an internal volume of the measuring chamber known from its dimensions, the internal volume of the pressure measuring cell can consequently be established in a reproducible manner.